1. Field of the Invention
The present invention relates to a rotating fan, and more particularly to a rotating fan having lightweight plastic blades.
2. Description of the Prior Art
Various rotating fans have been proposed in the prior art for providing a flow of air, particularly for cooling vehicle engines. It has been of particular concern to provide a fan having lightweight blades to reduce the effects of centrifugal and vibrational forces. It is also desirable to provide a fan which may be constructed relatively inexpensively.
A typical fan of the prior art has a spider and several blades stamped from a single sheet of steel. The center of the spider is provided with a hub for attaching the spider to a shaft. Such a fan must be constructed from relatively thick sheet metal to provide a ridigity. One consequence of using thick metal is that a relatively large portion of the fan's mass is distributed at some distance from the axis of rotation, resulting in a large moment of inertia for each blade. This necessitates careful balancing of the fan to avoid vibration. Relatively large centrifugal forces also result from the heavy blades, increasing the stress on the metal at the root of the blades and making failure from metal fatigue more likely.
One way of providing a lightweight fan to avoid many of the above problems is described in U.S. Pat. No. 3,628,888, issued to Wooden on Dec. 21, 1971. Wooden discloses a lightweight, inexpensive blade assembly including a fan spider having integral arms. The blades are formed by overlying a smaller thin metal plate adjacent a larger thin metal plate and spot-welding them together at the perimeter of the smaller plate. An extended arm of the fan's spider is received between the plates, with rivets holding the laminated assembly together.
Another approach to providing a lightweight fan involves the use of molded plastic blades. Examples of such fans are described in U.S. Pat. Nos. 3,260,312, issued to Elmer on July 12, 1966; 3,622,249, issued to Hayashi et al. on Nov. 23, 1971; and 3,751,181, issued to Hayashi on Aug. 7, 1973.
Elmer describes a fan wherein the blades are formed of a plastic material molded integrally about a central metal hub. Embedded in each blade is a metal spring member anchored to the hub, which provides controlled twisting of the blade and consequent automatic adjustment of the pitch of the blades as their rotational speed varies.
Hayashi et al. describes a fan assembly having an integrally molded plastic hub and blades. The plastic hub is provided with metal bushings for receiving bolts which secure the fan assembly to the driving member of a fluid coupling.
Hayashi describes a fan assembly having a hub with integral arms extending outwardly therefrom. Plastic blades are integrally molded about the arms and the hub. The arms are provided with holes into which the plastic of the blades extends to provide a firm bond between the blades and the arms.
Various other fan constructions have been proposed in the prior art. In U.S. Pat. No. 2,251,887, issued to Larsh on Aug. 5, 1941, there is described a flexible fan unit utilizing paper fabric fiber or rubber blades secured over a metallic reinforcing core. A rubber coating received upon metallic blades is described in U.S. Pat. No. 2,033,345, issued to Lee on Mar. 10, 1936. It is also known in the propeller field to construct aircraft propeller blades with a cylindrical metal root to which composite graphite fiber blades are attached, with the metal root being attached to a metal hub.